Parameterized Codebook with Subset Restrictions for use with Precoding MIMO Transmissions

ABSTRACT

One aspect of the teachings herein relates to signaling codebook restrictions, to restrict the precoder recommendations being fed back from a remote transceiver, so that precoder selections made by the remote receiver are restricted to permitted subsets of overall precoders within a defined set of overall precoders, or to permitted subsets within larger sets of conversion precoders and tuning precoders, for the case where the overall precoders are represented in factorized form by conversion and tuning precoders. As a non-limiting example, these teachings advantageously provide for precoder restrictions in LTE or LTE-Advanced networks, where ongoing development targets the use of larger, richer sets of precoders, and where the disclosed mechanisms for determining, signaling, and responding to subset restrictions provide significant operational advantages.

RELATED APPLICATIONS

This application claims priority from the U.S. provisional patentapplication filed on 7 Apr. 2010 and identified by Application No.61/321,679, and which is explicitly incorporated herein by reference.

FIELD OF THE INVENTION

The teachings herein generally relate to codebooks and precoding, andparticularly relate to the use of parameterized codebook subsets, suchas may be used to restrict codebook selections for differentMultiple-Input-Multiple-Output (MIMO) modes of operation.

BACKGROUND

Multi-antenna techniques can significantly increase the data rates andreliability of a wireless communication system. The performance is inparticular improved if both the transmitter and the receiver areequipped with multiple antennas, which results in a multiple-inputmultiple-output (MIMO) communication channel. Such systems and relatedtechniques are commonly referred to as MIMO.

The 3GPP LTE standard is currently evolving with enhanced MIMO support.A core component in LTE is the support of MIMO antenna deployments andMIMO related techniques. A current working assumption in LTE-Advanced isthe support of an 8-layer spatial multiplexing mode for 8 transmit (Tx)antennas, with the possibility of channel dependent precoding. Thespatial multiplexing mode provides high data rates under favorablechannel conditions.

With spatial multiplexing, an information carrying symbol vector s ismultiplied by an N_(T)×r precoder matrix W_(N) _(T) _(×r), which servesto distribute the transmit energy in a subspace of the N_(T)(corresponding to N_(T) antenna ports) dimensional vector space. Theprecoder matrix is typically selected from a codebook of possibleprecoder matrices, and typically indicated by means of a precoder matrixindicator (PMI). The PMI value specifies a unique precoder matrix in thecodebook for a given number of symbol streams.

If the precoder matrix is confined to have orthonormal columns, then thedesign of the codebook of precoder matrices corresponds to aGrassmannian subspace packing problem. In any case, The r symbols in thesymbol vector s each correspond to a layer and r is referred to as thetransmission rank. In this way, spatial multiplexing is achieved becausemultiple symbols can be transmitted simultaneously over the sametime/frequency resource element (TFRE). The number of symbols r istypically adapted to suit the current propagation channel properties.

LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink) andhence the received N_(R)×1 vector y_(n) for a certain TFRE on subcarriern (or alternatively data TFRE number n) is thus modeled by

y _(n) H _(n) W _(N) _(T) _(×r) s _(n) +e _(n)  (1)

where e_(n) is a noise/interference vector obtained as realizations of arandom process. The precoder, W_(N) _(T) _(×r), can be a widebandprecoder, which is constant over frequency, or frequency selective.

The precoder matrix is often chosen to match the characteristics of theN_(R)×N_(T) MIMO channel matrix H, resulting in so-called channeldependent precoding. This is also commonly referred to as closed-loopprecoding and essentially tries to focus the transmit energy into asubspace which is strong in the sense of conveying much of thetransmitted energy to the targeted receiver, e.g., a user equipment(UE). In addition, the precoder matrix also may be selected with thegoal of orthogonalizing the channel, meaning that after proper linearequalization at the UE or other targeted receiver, the inter-layerinterference is reduced.

In closed-loop precoding for the LTE downlink in particular, the UEtransmits, based on channel measurements in the forward link (downlink),recommendations to the eNodeB of a suitable precoder to use. A singleprecoder that is supposed to cover a large bandwidth (widebandprecoding) may be fed back. It also may be beneficial to match thefrequency variations of the channel and instead feed back afrequency-selective precoding report, e.g. several precoders, one perfrequency subband. This approach is an example of the more general caseof channel state information (CSI) feedback, which also encompassesfeeding back entities other than precoders, to assist the eNodeB inadapting subsequent transmissions to the UE. Such other information mayinclude channel quality indicators (CQIs) as well as a transmission rankindicator (RI).

For the LTE uplink, the use of closed-loop precoding means that theeNodeB selects precoder(s) and the transmission rank. The eNodeB maythereafter signal the selected precoder that the UE is supposed to useor alternatively apply precoding to the reference signals used forchannel estimation in the UE, thus avoiding the need of explicitsignaling. The eNodeB also may use certain bitmap-based signaling toindicate the particular precoders within a codebook that the UE isrestricted to using, see, e.g., Section 7.2 of the 3GPP TechnicalSpecification, TS 36.213. One disadvantage of such signaling is the useof bitmaps to indicate allowed or disallowed precoders. Codebooks withlarge numbers of precoders require long bitmaps, and the signalingoverhead associated with transmitting long bitmaps becomes prohibitive.

In any case, the transmission rank, and thus the number of spatiallymultiplexed layers, is reflected in the number of columns of theprecoder. Efficiency and transmission performance are improved byselecting a transmission rank that matches the current channelproperties. Often, the device selecting precoders is also responsiblefor selecting the transmission rank. One approach to transmission rankselection involves evaluating a performance metric for each possiblerank and picking the rank that optimizes the performance metric. Thesekinds of calculations are often computationally burdensome and it istherefore an advantage if calculations can be re-used across differenttransmission ranks. Re-use of calculations is facilitated by designingthe precoder codebook to fulfill the so-called rank nested property.This means that the codebook is such that there always exists a columnsubset of a higher rank precoder that is also a valid lower rankprecoder.

The 4-Tx House Holder codebook for the LTE downlink is an example of acodebook that fulfills the rank nested property. The property is notonly useful for reducing computational complexity, but is also importantin simplifying overriding a rank selection at a device other than theone that has chosen the transmission rank. Consider for example the LTEdownlink where the UE selects the precoder and rank, and conditioned onthose choices, computes a CQI representing the quality of the effectivechannel formed by the selected precoder and the channel. Since the CQIthus reported by the UE is conditioned on a certain transmission rank,performing rank override at the eNodeB side makes it difficult to knowhow to adjust the reported CQI to take the new rank into account.

However, if the precoder codebook fulfills the rank nested property,overriding the rank to a lower rank precoder is possible by selecting acolumn subset of the original precoder. Since the new precoder is acolumn subset of the original precoder, the CQI tied to the originalprecoder gives a lower bound on the CQI if the new reduced rank precoderis used. Such bounds can be exploited for reducing the CQI errorsassociated with rank override, thereby improving the performance of thelink adaptation.

Another issue to take into account when designing precoders is to ensurean efficient use of the transmitter's power amplifiers (PAs). Usually,power cannot be borrowed across antennas because, in general, there is aseparate PA for each antenna. Hence, for maximum use of the PAresources, it is important that the same amount of power is transmittedfrom each antenna, i.e., a precoder matrix W should fulfill

[WW*]_(mm)=κ,∀m.  (2)

Another equivalent way of formulating this is to notice that the rows ofW all need to have the same I²-norm, where the I²-norm of a row x withelements x_(k) is defined as

$\sqrt{\sum\limits_{k}{x_{k}}^{2}}.$

Thus, it is beneficial from a PA utilization point of view to enforcethis constraint when designing precoder codebooks and we hence refer to(2) as the PA utilization property.

Full power utilization is also ensured by the so-called constant modulusproperty, which means that all scalar elements in a precoder have thesame norm (modulus). It is easily verified that a constant modulusprecoder also fulfills the full PA utilization constraint in (2) andhence the constant modulus property constitutes a sufficient but notnecessary condition for full PA utilization.

As a further aspect of the LTE downlink and associated transmitteradaptation, the UE reports CQI and precoders to the eNodeB via afeedback channel. The feedback channel is either on the Physical UplinkControl Channel (PUCCH) or on the Physical Uplink Shared Channel(PUSCH). The former is a rather narrow bit pipe where CSI feedback isreported in a semi-statically configured and periodic fashion. On theother hand, reporting on PUSCH is dynamically triggered as part of theuplink grant. Thus, the eNodeB can schedule CSI transmissions in adynamic fashion. Further, in contrast to CSI reporting on PUCCH, wherethe number of physical bits is currently limited to 20, CSI reports onPUSCH can be considerably larger. Such a division of resources makessense from the perspective that semi-statically configured resourcessuch as PUCCH cannot adapt to quickly changing traffic conditions, thusmaking it important to limit their overall resource consumption.

More generally, maintaining low signaling overhead remains an importantdesign target in wireless systems. In this regard, precoder signalingcan easily consume a large portion of the available resources unless thesignaling protocol is carefully designed. The structure of possibleprecoders and the overall design of the precoder codebook plays animportant role in keeping the signaling overhead low. A particularlypromising precoder structure involves decomposing the precoder into twomatrices, a so-called factorized precoder. The precoder can then bewritten as a product of two factors

W _(N) _(T) _(×r) =W _(N) _(T) _(×k) ^((c)) W _(k×r) ^((t))  (3)

where an N_(T)×k conversion precoder W_(N) _(T) _(×k) ^((c)) strives forcapturing wideband/long-term properties of the channel such ascorrelation, while a kxr tuning precoder W_(k×r) ^((t)) targetsfrequency-selective/short-term properties of the channel.

Together, the factorized conversion and tuning precoders represent theoverall precoder W_(N) _(T) _(×r), which is induced by the signaledentities. The conversion precoder is typically, but not necessarily,reported with a coarser granularity in time and/or frequency than thetuning precoder to save overhead and/or complexity. The conversionprecoder serves to exploit the correlation properties for focusing thetuning precoder in “directions” where the propagation channel on averageis “strong.” Typically, this is accomplished by reducing the number ofdimensions k covered by the tuning precoder. In other words, theconversion precoder W_(N) _(T) _(×k) ^((c)) becomes a tall matrix with areduced number of columns. Consequently, the number of rows k of thetuning precoder W_(k×r) ^((t)) is reduced as well. With such a reducednumber of dimensions, the codebook for the tuning precoder, which easilyconsumes most of the signaling resources since it needs to be updatedwith fine granularity, can be made smaller while still maintaining goodperformance.

The conversion and the tuning precoders may each have a codebook oftheir own. The conversion precoder targets having high spatialresolution and thus a codebook with many elements, while the codebookthe tuning precoder is selected from needs to be rather small in orderto keep the signaling overhead at a reasonable level.

To see how correlation properties are exploited and dimension reductionachieved consider the common case of an array with a total of N_(T)elements arranged into N_(T)/2 closely spaced cross-poles. Based on thepolarization direction of the antennas, the antennas in the closelyspaced cross-pole setup can be divided into two groups, where each groupis a closely spaced co-polarized Uniform Linear Array (ULA) with N_(T)/2antennas. Closely spaced antennas often lead to high channel correlationand the correlation can in turn be exploited to maintain low signallingoverhead. The channels corresponding to each such antenna group ULA aredenoted H_(/) and H_(\), respectively. For convenience in notation, thefollowing equations drop the subscripts indicating the dimensions of thematrices as well as the subscript n. Assuming now that the conversionprecoder W^((c)) has a block diagonal structure,

$\begin{matrix}{W^{(c)} = {\begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}.}} & (4)\end{matrix}$

The product of the MIMO channel and the overall precoder can then bewritten as

$\begin{matrix}\begin{matrix}{{HW} = {\begin{bmatrix}H_{/} & H_{\smallsetminus}\end{bmatrix}W^{(c)}W^{(t)}}} \\{= {{\begin{bmatrix}H_{/} & H_{\smallsetminus}\end{bmatrix}\begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}}W^{(t)}}} \\{= {{\left\lbrack {\begin{matrix}{H_{/}{\overset{\sim}{W}}^{(c)}} & H_{\smallsetminus}\end{matrix}{\overset{\sim}{W}}^{(c)}} \right\rbrack W^{(t)}} = {H_{eff}{W^{(t)}.}}}}\end{matrix} & (5)\end{matrix}$

As seen, the matrix {tilde over (W)}^((c)) separately precodes eachantenna group ULA, thereby forming a smaller and improved effectivechannel H_(eff). If {tilde over (W)}^((c)) corresponds to a beamformingvector, the effective channel would reduce to having only two virtualantennas, which reduces the needed size of the codebook used for thesecond tuning precoder matrix W^((t)) when tracking the instantaneouschannel properties. In this case, instantaneous channel properties areto a large extent dependent upon the relative phase relation between thetwo orthogonal polarizations.

It is also helpful for a fuller understanding of this disclosure toconsider the theory regarding a “grid of beams,” along with DiscreteFourier Transform (DFT) based precoding. DFT based precoder vectors forN_(T) transmit antennas can be written in the form

$\begin{matrix}{{w_{n}^{({N_{T},Q})} = \begin{bmatrix}w_{1,n}^{({N_{T},Q})} & w_{2,n}^{({N_{T},Q})} & \ldots & w_{N_{T},n}^{({N_{T},Q})}\end{bmatrix}^{T}}{{w_{m,n}^{({N_{T},Q})} = {\exp \left( {j\; \frac{2\pi}{N_{T}Q}{mn}} \right)}},{m = 0},\ldots \mspace{14mu},\; {N_{T} - 1},{n = 0},\ldots \mspace{14mu},{{QN}_{T} - 1},}} & (6)\end{matrix}$

where w_(m,n) ^((N) ^(T) ^(,Q)) is the phase of the m:th antenna, n isthe precoder vector index (i.e., which beam out of the QN_(T) beams) andQ is the oversampling factor.

For good performance, it is important that the array gain function oftwo consecutive beams overlaps in the angular domain, so that the gaindoes not drop too much when going from one beam to another. Usually,this requires an oversampling factor of at least Q=2. Thus for N_(T)antennas, at least 2N_(T) beams needed.

An alternative parameterization of the above DFT based precoder vectorsis

$\begin{matrix}{{w_{l,q}^{({N_{T},Q})} = \begin{bmatrix}w_{1,{{Ql} + q}}^{({N_{T},Q})} & w_{2,{{Ql} + q}}^{({N_{T},Q})} & \ldots & w_{N_{T},{{Ql} + q}}^{({N_{T},Q})}\end{bmatrix}^{T}}{{w_{m,{{Ql} + q}}^{({N_{T},Q})} = {\exp \left( {j\frac{\; {2\pi}}{N_{T}}{m\left( {l + \frac{q}{Q}} \right)}} \right)}},}} & (7)\end{matrix}$

for m=0, . . . , N_(T)−1, l=0, . . . , N_(T)−1, q=0,1, . . . , Q−1, andwhere l and q together determine the precoder vector index via therelation n=Ql+q. This parameterization also highlights that there are Qgroups of beams, where the beams within each group are orthogonal toeach other. The q:th group can be represented by the generator matrix

$\begin{matrix}{G_{q}^{(N_{T})} = {\begin{bmatrix}w_{0,q}^{({N_{T},Q})} & w_{1,q}^{({N_{T},Q})} & \ldots & w_{{N_{T} - 1},q}^{({N_{T},Q})}\end{bmatrix}.}} & (8)\end{matrix}$

By ensuring that only precoder vectors from the same generator matrixare being used together as columns in the same precoder, it isstraightforward to form sets of precoder vectors for use in so-calledunitary precoding where the columns within a precoder matrix should forman orthonormal set.

Further, to maximize the performance of DFT based precoding, it isuseful to center the grid of beams symmetrically around the broad sizeof the array. Such rotation of the beams can be done by multiplying fromthe left the above DFT vectors w_(n) ^((N) ^(T) ^(,Q)) with a diagonalmatrix W_(rot) having elements

$\begin{matrix}{\left\lbrack W_{rot} \right\rbrack_{m\; m} = {{\exp \left( {j\; \frac{\pi}{{QN}_{T}}m} \right)}.}} & (9)\end{matrix}$

The rotation can either be included in the precoder codebook oralternatively be carried out as a separate step where all signals arerotated in the same manner and the rotation can thus be absorbed intothe channel from the perspective of the receiver (transparent to thereceiver). Henceforth, in discussing DFT precoding herein, it is tacitlyassumed that rotation may or may not have been carried out. That is,both alternatives are possible without explicitly having to mention it.

One aspect of the above-described factorized precoder structure relatesto lowering the overhead associated with signaling the precoders, basedon signaling the conversion and the tuning precoders with differentfrequency and/or time granularity. The use of a block diagonalconversion precoder is specifically optimized for the case of a transmitantenna array consisting of closely spaced cross-poles, but otherantenna arrangements exist as well. In particular, efficient performancewith a ULA of closely spaced co-poles should also be achieved. However,the method for achieving efficient performance in this regard is notobvious, with respect to a block diagonal conversion precoder structure.

Another aspect to consider is that, in a general sense, theabove-described factorized precoder feedback may prevent full PAutilization, and may violate the aforementioned rank nested property.These issues arise from the fact that the two factorized precoders—i.e.,the conversion precoder and the tuning precoder—are multiplied togetherto form the overall precoder and thus the normal rules for ensuring fullPA utilization and rank nested property by means of constant modulus andcolumn subset precoders, respectively, do not apply.

Further precoding considerations, particularly in the context of the LTEdownlink, include the fact that the PUCCH cannot bear as large a payloadsize as the PUSCH, for the previously described reasons. Thus, there isa risk of “coverage” problems when a UE reports CSI on the PUCCH. Inthis regard, it is useful to understand that current precoder designscommonly are optimized for transmissions to/from a single UE. In theMIMO context, this single-user context is referred to as a Single UserMIMO or SU-MIMO. Conversely, co-scheduling multiple UEs on the sametime/frequency resources is called Multi User MIMO or MU-MIMO. MU-MIMOis gaining increasing interest, but it imposes different requirements onprecoder reporting and the underlying precoder structures.

SUMMARY

One aspect of the teachings herein relates to signaling codebookrestrictions, to restrict the precoder recommendations being fed backfrom a remote transceiver, so that precoder selections made by theremote receiver are restricted to permitted subsets of precoders withinone or more larger sets. As a non-limiting example, these teachingsadvantageously provide for precoder restrictions in LTE or LTE-Advancednetworks, where ongoing development targets the use of larger, richersets of precoders, and where the disclosed mechanisms for determining,signaling, and responding to subset restrictions provide significantoperational advantages. In one embodiment, a codebook of overallprecoders is represented in factorized form by defined combinations ofconversion and tuning precoders, where restriction signaling restrictsoverall precoder selections by restricting one or both of conversion andtuning precoder selections.

Correspondingly, one embodiment disclosed herein comprises a method in awireless communication transceiver of controlling precoder selectionfeedback sent to another wireless communication transceiver, where theother transceiver precodes transmissions to the transceiver. As anon-limiting example, the transceiver comprises a mobile terminal orother type of user equipment (UE), and the other transceiver comprises asupporting base station in a wireless communication network. Theprecoder selection feedback indicates precoder selections by thetransceiver, which can be understood as precoder selectionrecommendations to be considered by the other transceiver in determiningthe precoding operation it uses for transmitting to the transceiver.

The method includes receiving restriction signaling from the othertransceiver that identifies one or more permitted subsets within adefined set of overall precoders, or, where the defined set of overallprecoders is represented by defined sets of conversion precoders andtuning precoders, the restriction signaling identifies one or morepermitted subsets of precoders within the defined sets of the conversionprecoders and tuning precoders. The method further includes generatingthe precoder selection feedback for sending to the other transceiverbased on restricting precoder selections by the transceiver according tosaid restriction signaling. Here, respective combinations of conversionand tuning precoders correspond to respective ones of the overallprecoders.

The permitted subset(s)—i.e., the precoders that are allowed forselection—can be changed dynamically, to reflect changes in operatingmodes, etc. In one example, there are two or more predetermined subsetsof conversion precoders and two or more predetermined subsets of tuningprecoders, and the restriction signaling identifies a permitted subsetof conversion precoders and a permitted subset of the tuning precoders.In other examples, the restriction signaling identifies the permittedsubset of conversion precoders without restricting tuning precoderselections, or vice versa. It also should be noted that there may be agreater or lesser number of tuning precoders and/or tuning precodersubsets than conversion precoders and/or conversion precoder subsets.Nor do the set/subset sizes of tuning precoders necessarily match thatused for conversion precoders.

Regardless, the conversion and tuning precoders operate as factorizedrepresentations of a set of overall precoders formed from definedcombinations of conversion and tuning precoders. This factorizedrepresentation is exploited herein in various advantageous ways, such asin how the restriction signaling is formed and processed, and in how theprecoder codebooks are structured, stored, and accessed in observance ofthe dynamically changeable precoder restrictions imposed by therestriction signaling.

As one example, the transceiver stores or otherwise maintains arepresentation of a conversion precoder codebook, representing a definedset of conversion precoders that are logically grouped into two or moresubsets, and likewise stores a tuning precoder codebook representing adefined set of tuning precoders grouped into two or more subsets. Eachprecoder represents a codebook entry that is identified by, for example,an index value. Thus, in one or more embodiments, the transceivergenerates the precoder selection feedback as indications of the indexvalues corresponding to the precoders it selects from the conversion andtuning precoder codebooks, with the understanding that such definedpairings represent in factorized form an overall precoder to beconsidered by the other transceiver.

Advantageously, the restriction signaling indicates conversion precoderrestrictions, tuning precoder restrictions, or both. Thus, the precoderselection feedback at any given time indicates a selected conversionprecoder, or a selected tuning precoder, or both, where those selectionsare restricted to the currently permitted subset(s). As such, therestriction signaling effectively restricts the transceiver's precodingrecommendations to those overall precoders that are formed from theconversion and tuning precoders that are candidates for selection by thetransceiver by virtue of their membership in the currently permittedsubset(s) of conversion and tuning precoders.

Advantageously, then, one or more codebooks of precoders can be“parameterized” in the sense that one or more predefined subsets withinsuch codebooks can be associated with one mode of operation or withcertain operating parameters, while one or more other predefined subsetscan be associated with another mode of operation or with certain otheroperating parameters, and codebook restrictions can be signaled simplyby signaling an indication of the mode or parameter(s) that are ineffect. A non-limiting example comprises splitting the conversion and/ortuning precoder codebook into one subset favored for SU-MIMO use and onesubset favored for MU-MIMO use.

With these non-limiting possibilities in mind, the method furthercomprises generating the precoder selection feedback for sending to theother transceiver. Particularly, the precoder selection feedback isgenerated based on restricting conversion and tuning precoder selectionsaccording to said restriction signaling. That is, the transceiverconsiders only those precoders that are candidates for selection byvirtue of their membership in the one or more subsets of precoders thatare indicated by the restriction signaling as being permitted forconsideration.

In complementary fashion, the teachings herein include a method for afirst transceiver to signal precoder selection restrictions to a secondtransceiver, with the understanding that the first transceiver receivesprecoder selection feedback from the second transceiver, to consider indetermining the precoding operation it applies for precodingtransmissions to the second transceiver. The first transceiver may be,by way of non-limiting example, a network base station, such as aneNodeB in an LTE or LTE-Advanced network. Correspondingly, the secondtransceiver comprises a user terminal or other such user equipment (UE).

In one embodiment, the method includes the first transceiver determininga restriction that limits precoder selection by the second transceiverto one or more permitted subsets of precoders within defined sets ofconversion precoders and tuning precoders. The method further includesgenerating restriction signaling for indicating the one or morepermitted subsets to the second transceiver, which may store the definedsets of conversion and tuning precoders as conversion and tuningprecoder codebooks. The restriction(s) thus can be understood aslimiting conversion and/or tuning precoder selections by the secondtransceiver to permitted subset(s) within the conversion and/or tuningprecoder codebooks. In effect, then, this allows the first transceiverto limit selections of an overall precoder by the second transceiver toa given subset of overall precoders within a codebook of overallprecoders.

To make the restrictions active at the second transceiver, the firsttransceiver sends restriction signaling to the second transceiver, torestrict precoder selection by the second transceiver to the one or morepermitted subsets. Again, the conversion and tuning precoders representa set of overall precoders in factorized form, such that precoderrecommendations by the second transceiver can be restricted to a desiredsubset of the overall precoders based on indicating correspondingselection restrictions for the conversion and/or tuning precoders.

In a non-limiting example, the first transceiver, which again may benetwork base station of some type, operates in a SU-MIMO mode at certaintimes, and operates in an MU-MIMO mode at certain other times. Certainones of the overall precoders or, equivalently, certain ones of theconversion and/or tuning precoders, are predefined as being associatedwith the SU-MIMO mode of operation and certain other ones are predefinedas being associated with the MU-MIMO mode of operation. Thus, as thefirst transceiver dynamically switches between SU-MIMO and MU-MIMO modesof operation, it uses the restriction signaling to identify which modeis active. That indication effectively identifies the permittedsubset(s) of precoders to the second transceiver as it is configured toassociate one or more precoder subsets with SU-MIMO operation and one ormore other precoder subsets with MU-MIMO operation.

Similarly, the precoders can be grouped into different subsets accordingto one or more other parameters. For example, one subset of theprecoders may include only precoders that satisfy a full PA utilizationproperty for power amplifier utilization at the first transceiver.Another subset of precoders does not satisfy the full PA utilizationproperty. In this manner, when the first transceiver prioritizes full PAutilization, it uses its restriction signaling to identify as thepermitted subset(s) only those precoders that satisfy the full PAutilization property. Otherwise, the restriction signaling can be usedto identify as the permitted subset(s) those precoders that do notsatisfy the property. Such control can be effected, for example, bysetting or clearing a flag conveyed by the restriction signaling, wherethe second transceiver is configured to recognize the state of that flagas indicating which precoder subset(s) are permitted.

Of course, the present invention is not limited to the above briefsummary of features and advantages. Other features and advantages willbe recognized from the following detailed discussion of exampleembodiments and from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of example embodiments of a first transceiverthat is configured to transmit precoded transmissions to a secondtransceiver.

FIG. 2 is a diagram of one embodiment of logical subsets of precoderswithin a larger codebook of precoders.

FIG. 3 is a diagram of one embodiment of logically separating precoderswithin a codebook into currently permitted and currently non-permittedsubsets, to restrict precoder selection recommendations.

FIG. 4 is a diagram of one embodiment of logically separating precoderswithin a codebook into first and second pluralities, such as where thefirst plurality of precoders is associated with one operating mode andthe second plurality of precoders is associated with another operatingmode.

FIG. 5 is a diagram of one embodiment of a signaling structure used forefficiently signaling precoder restrictions.

FIG. 6 is a diagram of one embodiment of logically separating precoderswithin a codebook into different types of precoders.

FIG. 7 is a diagram of one embodiment of storing/maintaining precodercodebooks, where each codebook is used to maintain a different type ofprecoder.

FIG. 8 is a diagram of one embodiment of maintaining factorizedprecoders, wherein a recommended precoder is defined or formed as thematrix multiplication of a selected pair of the factorized precoders.

FIG. 9 is a logic flow diagram illustrating one embodiment of a methodof precoder feedback generation at a second transceiver, responsive torestriction signaling from a first transceiver.

FIG. 10 is a logic flow diagram illustrating one embodiment of a methodof precoder selection restriction and signaling at a first transceiver,to restrict the precoder recommendations made by a second transceiverreceiving precoded transmissions from the first transceiver, to one ormore currently permitted subsets within a larger set of precoders.

FIG. 11 is a block diagram of one embodiment of a precoding circuit,such as may be implemented in the first transceiver of FIG. 1.

FIG. 12 is a block diagram of one embodiment of a wirelesscommunication, wherein precoding restriction signaling and precoderselection restrictions as taught herein are used between a base stationand an item of user equipment (a “UE”).

DETAILED DESCRIPTION

FIG. 1 depicts a first wireless communication transceiver 10 and asecond wireless communication transceiver 12, referred to forconvenience as transceivers 10 and 12. The transceiver 10 includes anumber of antennas 14 and associated transceiver circuits 16 (includingone or more radiofrequency receivers and transmitters), along withcontrol and processing circuits 18. At least functionally, the controland processing circuits 18 include a precoding controller 20, a feedbackprocessor 22, and one or more memory circuits 24 that maintain a set 26of overall precoders 28. The transceiver 10 in one or more embodimentsmaintains a codebook representation of the defined set 26 of overallprecoders 28, and this disclosure thus equivalently refers to the set 26as the “codebook” 26. Here, a “codebook representation” comprises, forexample, a stored set of precoding entries that are accessed orotherwise identified according to predefined index values—e.g., there ispredetermined mapping of precoder entries to codebook index positions.

It is also contemplated herein that the set 26 of precoders 28 may bemaintained in another, larger codebook of precoder entries, which mayinclude other types of precoders and/or precoders having propertiesdifferent from the overall precoders 28 in the set 26. In at least oneembodiment, at least some of the overall precoders 28 are DFT-basedprecoders that provide for beamforming of transmissions by thetransceiver 10, where each such precoder represents a unique combinationof one “conversion” precoder and one “tuning” precoder. That is, infactorized form, each such overall precoder 28 is formed, e.g., by thematrix multiplication of one conversion precoder and one tuningprecoder. Thus, the set 26 of overall precoders 28 may be alternately(and equivalently) represented as sets of conversion and tuningprecoders.

As for the set 26, it will be understood that it comprises, for example,a “codebook” of entries, with each entry representing one of theprecoders 28 in the set 26. The number “28” is used generally as areference in the both the singular and plural senses, for referring toone or multiple precoders 28. Suffix designations are used, too, wherehelpful for clarity, e.g., precoder 28-1, precoder 28-2, and so on. Asused in this sense, “precoder” means a matrix of vector ofantenna-weighting values to be applied for transmitting a signal from aset of transmit antenna ports.

The second transceiver 12 includes a number of antennas 30 andassociated transceiver circuits 32 (including one or more radiofrequency receivers and transmitters), along with control and processingcircuits 34. At least functionally, the control and processing circuits34 include received signal processing circuitry 36, e.g.,demodulation/decoding circuits, and further include one or moreestimation circuits 38, for estimating channel conditions and/or signalquality. Because the ability of the second transceiver 12 to receivespatially-multiplexed or other precoded transmissions from the firsttransceiver 10 depends on propagation channel conditions, thetransceiver 12 uses its evaluation of signals received from the firsttransceiver 10 to estimate channel conditions, and those estimatesprovide a basis for the second transceiver 12 making precoderrecommendations to the first transceiver 10.

In support of such functionality, the control and processing circuits 34include one or more memory circuits 40, and a precoding feedbackgenerator 42. The memory circuit(s) 40 store, for example, the samecodebook 26 of precoders 28 as stored at the transceiver 10. In thismanner, the transceiver 12 can send precoder selection feedback 44 tothe transceiver 10 by sending PMI values. The PMI values indicate thecodebook index value of the precoder(s) 28 selected by the transceiver12, representing the precoder(s) 28 recommended by the secondtransceiver 12, for use by the first transceiver 10 in precodingtransmissions to the second transceiver 12. These recommendations changedynamically, such as in response to changing channel conditions betweenthe first and second transceivers 10 and 12.

To constrain or otherwise restrict the precoder selections made by thetransceiver 12, the transceiver 10 transmits restriction signaling 48.Advantageously, the teachings herein disclose various approaches toparameterizing the set 26 of overall precoders 28, wherein one or moresubsets of precoders 28 within the codebook 26 are viewed as being“permitted” for use when a given parameter or parameters take on acertain value, while other one(s) of the precoder subsets are consideredas being “permitted” when the parameter values change. As an exampleintroduction, the parameter of interest may be the transmission mode ofthe transceiver 10, wherein the mode has two values: SU-MIMO andMU-MIMO. One subset of precoders 28 in the codebook 26 is favored foruse with SU-MIMO, while another subset is favored for use with MU-MIMO.Thus, the value of a transmission mode parameter—e.g., set when MU-MIMOis active and cleared when SU-MIMO is active—may be used to indicatewhich precoder subset(s) are permitted for use within the codebook 26.

With regard to this non-limiting example, one sees that the approachadvantageously provides reduced signaling overhead. That is, therestriction signaling 48 may comprise a flag or other logical indicator,however, it is not so limited and may use other formats and includeother information. In any case, the restriction signaling 48 should beunderstood as indicating to the transceiver 12 which subset(s) ofprecoders 28 in its codebook are permitted for selection as precodingrecommendations to the transceiver 10. Moreover, in one or moreembodiments, the transceiver 12 is advantageously configured to use thesame signaling format for its precoder selection feedback 44 regardlessof whether subset restrictions have been imposed on its precoderselections, and regardless of the particulars of any such restrictions.Among other things, keeping the same signaling format across changingrestrictions simplifies processing and transmission of the precoderselection feedback 44 by the transceiver 12, simplifies reception andprocessing of precoder selection feedback 44 by the transceiver 10, andprovides for a consistent signaling overhead.

In one or more embodiments, the control and processing circuits 18 ofthe transceiver 10 at least in part comprise computer-based circuitry,e.g., one or more microprocessors and/or digital signals processors, orother digital processing circuitry. In at least one embodiment, suchcircuitry is specially configured to implement the methods taught hereinfor the transceiver 10, based on executing stored computer programinstructions, such as may be stored in the memory circuit(s) 24.Likewise, in at least one embodiment, the control and processingcircuits 34 at the second transceiver 12 are implemented at least inpart via programmable digital processing circuitry. For example, thecontrol and processing circuits 34 in one or more embodiments includeone or more microprocessors or digital signal processors configured toimplement at least a portion of the methods taught herein for thetransceiver 12, based on executing computer program instructions storedin the one or more memory circuits 40.

With these example implementation details in mind, the transceiver 12 isconfigured to control precoder selection feedback 44 sent to anothertransceiver 10, where the other transceiver 10 precodes transmissions 46to the transceiver 12. The precoder selection feedback 44 indicatesprecoders 28 dynamically selected by the second transceiver 12 from adefined codebook 26 of precoders 28. The transceiver 12 includes amemory 40 configured to store the codebook 26 of precoders 28. Further,as noted, the transceiver 12 includes a precoding feedback generator 42,which is configured to manage the codebook 26 as two or morepredetermined subsets of precoders, wherein at least one predeterminedsubset includes more than one of the precoders 28 in the codebook 26.That is, at least one of the subsets is not a set of one.

As an example, refer momentarily to FIG. 2 where an example codebook 26includes a plurality of precoders 28, individually depicted as 28-1,28-2, 28-3, and so on. As will be understood by those of ordinary skillin the art, each precoder 28 is, for example, a matrix of numericalvalues corresponding to transmit antenna signal weights. One sees thatthere are a total of N precoders 28 in the example illustration, withthose N precoders 28 subdivided into a number of subsets 50. At leastone subset 50 includes more than two of the precoders 28. By way ofnon-limiting example, the N precoders 28 are divided into two subsetsreferred to as 50-1 and 50-2. For illustration, the subset 50-1 includesat least the precoders 28 identified as 28-1, 28-2, and 28-3, which arealso identified as “ENTRY 1,” “ENTRY 2” and “ENTRY 3” in the drawing.The subset 50-2 includes at least the precoders 28 referred to 28-(N−1)and 28-N, which are also identified as “ENTRY N−1”, and “ENTRY N.”

Thus, in this example instance, each subset 50-1 and 50-2 includes morethan one precoder 28 in the overall set of N precoders 28.Advantageously, the transceiver 10 is configured to generate restrictionsignaling 48 that efficiently indicates which one(s) of the subsets 50are currently permitted for use by the transceiver 12 in generating theprecoder selection feedback 44. For example, one subset 50 might beassociated with one operating mode, while another subset 50 might beassociated with another operating mode. The transceiver 10 can simplyset or clear a flag or mode indicator to indicate which subset 50currently is to be considered as the “permitted” subset for precoderselections by the transceiver 12.

Correspondingly, the receiver in the transceiver circuits 32 of thetransceiver 12 is configured to receive signaling from the transceiver10, including the restriction signaling 48. As noted, the restrictionsignaling 48 indicates which one or more of the predetermined subsets 50are currently permitted subsets 52 (as shown in FIG. 3) for use by saidtransceiver 12 in determining the precoder selection feedback 44. Here,the precoding feedback generator 42 is configured to restrict precoderselection for the precoder selection feedback 44 to those precoders 28in the currently permitted subsets 52. Conversely, the remaining subsetsamong the defined subsets 50 would be considered as non-permittedsubsets 54, and the individual precoders 28 having membership in thenon-permitted subsets 54 would therefore be excluded from considerationin the generation of the precoder selection feedback 44.

In one embodiment, and with reference to FIG. 4, the transceiver 12 isconfigured to manage the codebook 26 as two or more predeterminedsubsets 50 by associating a first plurality 56 of precoders 28 in thecodebook 26 with a first operating mode and associating a secondplurality 58 of precoders 28 in the codebook 26 with a second operatingmode. As a non-limiting example, the first plurality 56 of precoders 28is intended for use in a particular operating mode, while the secondplurality 58 of precoders 28 is intended for use in a differentoperating mode. The first mode is, for example, SU-MIMO, while thesecond mode is MU-MIMO. In at least one embodiment where the subsets 50of precoders 28 are grouped according to modes, the restrictionsignaling 48 comprises a mode indicator that indicates the operatingmode that is or will be active, and thus indicates which mode appliesfor the selection of precoders 28 from the codebook 26.

With reference to FIG. 5, one sees an embodiment of the restrictionsignaling 48, wherein the signaling comprises a message identifier (ID)60 and an indicator 62. As an example, the message ID 60 is a uniquenumeric value associated with restriction signaling, and the transceiver12 is configured to recognize the restriction signaling 48, based onrecognizing the identifier 60. In at least one embodiment, the indicator62 comprises a mode indicator. For example, the mode indicator may be asingle bit that is set to indicate that one mode is active, or clearedto indicate that another mode is active. The transceiver 12 in such anembodiment would restrict its precoder selections (recommendations) tothose precoders 28 that are logically associated with the indicatedmode—i.e., to those precoders 28 that are permitted for the indicatedmode.

Of course, to the extent that more than two modes are defined, theindicator 62 may comprise a multi-bit binary value that indicates whichone (or ones) of the defined modes are active. Similarly, in one or moreembodiments, there are N precoders 28 in total in the codebook 26, whereN is an integer>2. Further, there are M predetermined subsets 50 definedwithin the overall set of N precoders 28, where M<N. With thisrelationship, at least one subset 50 (e.g., subset 50-1 or 50-2)includes more than one precoder 28. Correspondingly, the restrictionsignaling 48 in one or more embodiments includes an M-length bit maskthat indicates which ones of the M predetermined subsets 50 arecurrently permitted subsets 52. Alternatively, the restriction signaling48 includes one or more binary values, e.g., the indicator 62 is formedas one or more binary values, where each value indicates which one orones of the subsets 50 are to be treated by the transceiver 12 ascurrently permitted subsets 52.

Thus, as seen in FIG. 6, the codebook 26 in one or more embodiments canbe regarded as dividing into at least a first type 64 of precoders 28and a second type 66 of precoders 28. Both types may be representedusing similar matrix/vector structures, but they may have values thatyield different operating characteristics for the transceiver 10, orvalues that are optimized for certain operating conditions at thetransceiver 10. Thus, the transceiver 10 would restrict precoderselections by the transceiver 12 to whichever precoder type best suitsthe current operating conditions or mode.

Further, note that in one or more embodiments herein the precoders 28 inthe codebook 26 stored at the transceiver 10 are regarded as andhereafter referred to as “overall” precoders 28 in the sense that theycan be understood as representing the combination of a selectedconversion precoder and a selected tuning precoder, which were discussedin example form earlier herein. With this understanding, a selectedconversion precoder and a selected tuning precoder are understood as a“factorized” representation of a selected overall precoder 28, in thesense that, e.g., the matrix product of the selected conversion andtuning precoders forms the corresponding selected overall precoder 28.

In this manner, it is equivalent to restrict precoder selections todefined subsets of overall precoders 28, or to restrict precoderselections to defined subsets of the conversion and tuning precodersrepresenting the factorized form of the overall precoders 28. In thissense, the transceiver 10 and/or the transceiver 12 may store theoverall precoders 28 in the codebook 26, or the codebook 26 may bestructured as two codebooks, one containing conversion precoders and onecontaining tuning precoders. The use of factorized conversion and tuningprecoder codebooks, and the use of restriction signaling 48 thatindicates conversion and/or tuning precoder restrictions offers a numberof advantages, in terms of being able to flexibly define the subsetrestrictions and in terms of being able to efficiently signal suchrestrictions.

An example arrangement appears in FIG. 7, where the codebook 26 isdepicted as comprising a conversion precoder codebook 70 of conversionprecoders 74, and a tuning precoder codebook 72 of tuning precoders 76.FIG. 8 illustrates that the plurality of conversion precoders 74 atleast logically may be regarded as a larger, defined set 80 ofconversion precoders 74 that is subdivided into two or more subsets 82.Likewise, the plurality of tuning precoders 76 may be regarded as alarger, defined set 86 of tuning precoders that is subdivided into twoor more subsets 88. Note that the subset sizes or numbers may notnecessarily be equal between the codebooks. It will be appreciated then,that the transceiver 10 may maintain a codebook or set 26 of precoders28, where at least some number of those precoders 28 are “overall”precoders, each representing a unique combination of one conversionprecoder 74 and one tuning precoder 76. Thus, precoder restrictions maybe generated and signaled in terms of which ones of the overallprecoders are permitted for use, or, equivalently, in terms of whichones of the conversion precoders 74 and/or tuning precoders 76 arepermitted for use.

For example, the restriction signaling 48 from the transceiver 10 can begenerated and signaled dynamically as needed, to indicate which ones ofthe subsets 82 and/or the subsets 88 are to be considered by thetransceiver 12 as “permitted” for use in generating the precoderselection feedback 44. The illustration shows that one subset 82 ofconversion precoders 74 is a permitted subset 84-1 and that one subset88 of tuning precoders 76 is a permitted subset 84-2. It will beunderstood that restrictions may be applied to the subsets 82 ofconversion precoders 74, the subsets 88 of tuning precoders 76, or both.The particular manner in which restriction is applied will depend uponthe factorization details applicable to the conversion and tuningprecoders 74 and 76.

As noted, the transceiver 12 may be a UE or other type of wirelesscommunication device, and the transceiver 10 may be an eNodeB in an LTEor LTE-Advance network, or may be another type of wireless communicationnetwork base station. In at least one such embodiment, the transceiver12 is configured to receive said restriction signaling 48 as RadioResource Control (RRC) layer signaling. Also note that the transceiver12 in one or more embodiments does not necessarily operate with precoderselection restrictions. For example, in one embodiment, the precodingfeedback generator 42 is configured to use or not use subsetrestrictions in its precoder selections, in dependence on therestriction signaling 48. For example, there is a defined signalingvalue or pattern that indicates whether restriction should be used, orthe absence of an explicitly signaled restriction value is taken to meanthat restriction is not in use.

With the above possibilities for operation of the transceiver 12 inmind, FIG. 9 illustrates a method 900 carried out in the transceiver 12with respect to the transceiver 10. The illustrated method 900 includesreceiving (Step 902) restriction signaling 48 from the transceiver 10that identifies one or more permitted subsets 50 within a defined set 26of overall precoders 28, or, where the defined set 26 of overallprecoders 28 is represented by defined sets 80, 86 of conversionprecoders 74 and tuning precoders 76, the restriction signaling 48identifies one or more permitted subsets 84 of precoders 74, 76 withinthe defined sets 80, 86 of said conversion precoders 74 and tuningprecoders 76. The method 900 further includes generating (Step 904) saidprecoder selection feedback (44) for sending to the other transceiver(10) based on restricting (Step 906) precoder selections by thetransceiver (12) according to the restriction signaling (48).

Thus, the restriction signaling 48 may be generated and transmitted bythe transceiver 10, in explicitly in terms of the precoders 28 withinthe defined set 26—i.e., to identify a permitted subset 50 of suchprecoders 28. Alternatively but equivalently, the transceiver 10 maygenerate and transmit the restriction signaling 48 in terms of permittedsubsets 84 of precoders 74, 76 within the defined sets 80, 86 ofconversion precoders 74 and tuning precoders 76. More particularly,conversion and/or tuning precoder selection may be restricted, toachieve an equivalent restriction on selection of precoders 28.Moreover, in one or more embodiments, the restriction signaling 48identifies a permitted subset or subsets 50 of (overall) precoders 28,and the transceiver 12 maps or otherwise translates such restrictionsinto conversion and/or tuning precoder selection restrictions.

On that point, as noted, respective combinations of conversion andtuning precoders 74, 76 correspond to respective ones of the overallprecoders 28. That is, each such overall precoder 28 represents a uniquecombination of one conversion precoder 74 and one tuning precoder 76.Correspondingly, in at least one embodiment of the method 900, aconversion precoder codebook 70 contains the defined set 80 ofconversion precoders 74 and a tuning precoder codebook 72 contains thedefined set 86 of tuning precoders 76, and restriction signaling 48indicates at least one of: (a) a permitted subset 84-1 of conversionprecoders 74 in the conversion precoder codebook 70 and (b) a permittedsubset 84-2 of tuning precoders 76 in said tuning precoder codebook 72.

It will be understood that the precoder selection feedback 44 maysimultaneously indicate the selected conversion and tuning precoders, orit may indicate the selected conversion precoder in some instances andthe selected tuning precoder in other instances. As an example of this,the conversion precoder selection interval is slower than the tuningprecoder selection interval—i.e., tuning precoder selections are updatedmore frequently than conversion precoder selections. Thus, the precoderselection feedback 44 need not indicate conversion precoder selectionsas frequently as tuning precoder selections. Similarly, the precoderselection feedback 44 may be sent on different channels or even protocollayers. In other variations, a conversion precoder selection covers arelatively wide frequency band, which is subdivided into narrowersub-bands and respective tuning precoder selections are signaledrespectively for each sub-band.

Turning to the transceiver 10, FIG. 10 illustrates a method 1000,wherein the precoding controller 20 of the transceiver 10 determines(Step 1002) a restriction that limits precoder selection by saidtransceiver 12 to one or more permitted subsets 84 of precoders 74, 76within defined sets 80, 86 of conversion precoders 74 and tuningprecoders 76. The method also includes generating (Step 1004)restriction signaling 48 for indicating the one or more permittedsubsets (84) to the transceiver 12, and sending (Step 1006) therestriction signaling 48 to the other transceiver 12. Doing so serves torestrict precoder selection by the transceiver 12 to said one or morepermitted subsets (84), so that the precoding recommendations made bythe transceiver 12 are consistent with the restrictions.

The precoding controller 20 is further configured to generate therestriction signaling 48 to indicate the restriction. It will beunderstood that this may be a dynamic process, where the restrictionsignaling 48 is updated as needed, to reflect changing restrictions. Atransmitter in the transceiver circuits 16 of the transceiver 10 iscooperatively associated with precoding controller 20, and is configuredto transmit the restriction signaling 48.

In FIG. 11, one sees an example embodiment of precoding circuitry 90,such as is implemented on or in conjunction with the precodingcontroller 20 and transceiver circuit 16 of the transceiver 10. Theillustrated circuitry is used for generating the precoded transmissions46, for transmission to the transceiver 12. Of course, othertransceivers besides transceiver 12 may be supported. The precodingcircuitry 90 includes layer processing circuitry 92, which forms symbolvectors of length s, for transmit layer in use for MIMO transmission bythe transceiver 10, where the precoding controller 20 sets the transmitrank. The circuitry further includes a precoder 94, which applies theprecoding operations used for generating precoded transmissions from theantennas 14 of the transceiver 10, including the precoded transmissions46 to the transceiver 12. Note that the precoding operation applied forprecoding transmissions to the transceiver 12 is determined inconsideration of the precoder selection feedback 44 from the transceiver12, but does not necessarily follow that feedback.

Notable in the illustration is Inverse Fast Fourier Transform (IFFT)processing circuitry 96 feeding into a plurality of transmit antennaports 98. In one or more embodiments taught herein, each conversionprecoder 74 comprise a block diagonal matrix, where each block comprisesa DFT-based precoder that provides for a number of beams, forbeamforming from a subset of the N_(T) antenna ports 98. In at least onesuch embodiment, the defined set 80 of conversion precoders 74 includesN_(T)Q different conversion precoders 74 and the defined set 86 oftuning precoders 76 includes a number of corresponding tuning precoders76.

Each conversion precoder 74 comprises a block diagonal matrix in whicheach block comprises a DFT-based precoder that defines N_(T)Q differentDFT based beams for a subgroup in the group of N_(T) transmit antennaports 98 at the transceiver 10, where Q is an integer value and wherethe N_(T)Q different conversion precoders 74, together with one or moreof the tuning precoders 76, correspond to a set of N_(T)Q differentoverall precoders 28. Each overall precoder 28 thus represents asize-N_(T) DFT-based beam over the group of N_(T) transmit antennasports (98).

Turning to FIG. 12, one sees that the transceiver 10 may be implementedas part of wireless communication network 100, which includes a RadioAccess Network (102), including one or more base stations 104. Here, itwill be understood that the base station 104 represents the transceiver10, but may include additional interface and processing circuitry notdiscussed thus far. However, as such circuitry is not germane to thisdiscussion and, in any case, is generally well understood in the basestation design arts, the overall architecture of the base station 104 isnot detailed.

One also sees in FIG. 12 an item of user equipment, i.e., UE 106, whichmay be understood as representing a particular embodiment of thetransceiver 12. As with the base station 104, the UE 106 includesinterface and processing circuitry not previously described herein.However, as such circuitry is well understood in a general sense and isnot germane to the discussion, it is not further detailed here. One alsosees that the RAN 102 is communicatively coupled to a Core Network (CN)108, the implementation of which will depend upon the wirelesscommunication standards at issue, e.g., it may be an Evolved Packet Core(EPC) in an LTE/LTE-Advanced implementation. The CN 108 generally iscommunicatively coupled to one or more external networks 110, such asthe Internet and allows call and data delivery to/from multiple UEs 106,which are supported by radio links to the RAN 102.

The base station 104 is configured to transmit precoded transmissions tothe illustrated UE 106, based at least in part on receiving precoderselection feedback 44 from the UE 106. In particular, however, the UE'sselection of precoders for recommendation to the base station 104 isrestricted according to its receipt of restriction signaling 48. In thiscontext, the base station 104 stores a codebook 26 of overall precoders28. The base station 104 alternatively organizes its codebook 26 as aconversion precoder codebook 70 and a tuning precoder codebook 72, whereindividual pairings of the conversion and tuning precoders 74 and 76represent respective overall precoders 28 in factorized form.

Further in the illustrated arrangement, the UE 106 stores the same orequivalent codebooks, so that its indications of precoder selections areunderstood by the base station 104. For example, UE 106 stores aconversion precoder codebook 70 and a tuning precoder codebook 72. Thebase station 104 stores copies of the same codebook, or, as mentioned,it stores a codebook 26 of overall precoders 28 that correspond to thecodebooks stored by the UE 106. In any case, the UE 106 is configured torespond to restriction signaling 48 received from the base station 106,by restricting its precoder recommendations to those precoders that arein currently permitted subsets, as indicated by the restrictionsignaling.

As for the base station's precoding in particular, and for precoding bythe transceiver 10 in a more general example, one may turn back to theprecoding circuit 90 of FIG. 11. The layer processing circuitry 92converts a stream of input data, e.g., an input symbol stream, into oneor more symbol vectors s. As explained, the precoding controller 20provides a rank control signal to the layer processing circuitry 82, tocontrol the number of layers to which the input data is mapped. Thesymbol vector(s) s are input to precoder 94, which applies a precodingoperation. For example, it forms an overall precoder 28, denoted as W,as the matrix multiplication of a selected conversion precoder 74,denoted as W^((c)), and a selected tuning precoder 76, denoted asW^((t)). This precoding operation may or may not follow therecommendations represented by the precoder selection feedback 44, butthe precoding controller 20 is configured to consider that feedback indetermining the currently applied precoding operation. The precodedstream(s) are output to the IFFT circuitry 96, which applies an IFFT tothe streams. After that transformation, the steams are directed torespective ones in a set of antenna ports 98, for transmission from thetransmitter's antennas 14.

In at least one embodiment, the transceiver 10 uses Discrete FourierTransform (DFT) based precoders implementing a partially overlappinggrid of beams. This approach is suitable for closely spaced co-polarizedantennas such as a Uniform Linear Array with N_(T) elements. Thus, itwill be understood that in one or more embodiments, the overallprecoders 28 in the codebook 26 include a number of DFT-based precoders.For example, the overall precoders 28 may be represented by a number ofDFT-based conversion precoders 74 and associated tuning precoders 76,such as illustrated in FIG. 8.

DFT based precoders are also suitable for the two N_(T)/2 elementantenna group ULAs in a closely spaced cross-pole setup. By a cleverchoice of the codebook entries for the conversion and tuning precoders74 and 76 and by exploiting them jointly, the teachings herein ensurere-use of the DFT based size N_(T)/2 precoders for antenna group ULAsalso in forming the needed number of DFT based size N_(T) precoders foran N_(T) element ULA. Moreover, one or more embodiments disclosed hereinprovide a structure for the conversion precoders 74 that allows re-usingexisting codebooks with DFT based precoders and extending their spatialresolution.

Further, in at least one embodiment, it is proposed herein to use aprecoder structure which solves the problems related to PA utilizationand rank nested property for a factorized precoder design—e.g., in thecase where an overall precoder W is represented in factorized form by aconversion precoder 74 and a tuning precoder 76. By using a so-calleddouble block diagonal tuning precoder 76 combined with a block diagonalconversion precoder 74, full PA utilization is guaranteed and rankoverride exploiting nested property also for the overall precoder ispossible. However, it should be kept in mind that these and otherspecial precoder types and structures may be represented in subsets orgroups within a larger number of precoders and that systems configuredaccording to the teachings herein may use codebook having additionalentries that do not conform to some of the specialized block diagonaland other forms described herein.

In any case, an example embodiment herein allows DFT based precoderelements for an antenna group ULA in a closely spaced cross-pole to bereused for creating a grid of beams with sufficient overlap for a ULA oftwice the number of elements compared with the antenna group ULA. Inother words, the overall precoders 28 in the codebook 26 can be designedfor use with the multiple antennas 14 of the transceiver 10, regardlessof whether those antennas 14 are configured and operated as an overallULA of N_(T) antennas or antenna elements, or as two cross-polarized ULAsub-groups, each having N_(T)/2 antennas or antenna elements.

Consider the block diagonal factorized precoder design given as

$\begin{matrix}{{W = {{W^{(c)}W^{(t)}} = {\begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}W^{(t)}}}},} & (10)\end{matrix}$

where W=an overall precoder 28 and W^((c)) and W^((t)) represent theconversion/tuning precoder pairing from which W is formed. Also, notethat in order to tailor the transmission to ±45 degrees cross-poles, thestructure of each conversion precoder 74 can be modified by means of amultiplication from the left with a matrix

$\begin{matrix}{\begin{bmatrix}I & {I\; ^{j\; \varphi}} \\I & {{- I}\; ^{j\; \varphi}}\end{bmatrix},} & (11)\end{matrix}$

which, for φ=0, rotates the polarizations 45 degrees to align withhorizontal and vertical polarization. Other values of φ may be used toachieve various forms of circular polarization. Henceforth, it isassumed for purposes of this discussion that such rotations are absorbedinto the channel.

For an N_(T) element ULA, the precoder W for rank 1 is to be a N_(T)×1vector as

$\begin{matrix}{W = {w_{n}^{({N_{T},Q})} = {\begin{bmatrix}w_{1,n}^{({N_{T},Q})} & w_{2,n}^{({N_{T},Q})} & \ldots & w_{N_{T},n}^{({N_{T},Q})}\end{bmatrix}^{T}.}}} & (12)\end{matrix}$

In this context, recall that W is may be formed as the product (matrixmultiplication) of a given conversion precoder 74 and a correspondingtuning precoder 76, e.g., W=W^((c))W^((t)). Noting that for antennasm=0, 1, . . . , N_(T)/2−1,

$\begin{matrix}\begin{matrix}{w_{m,n}^{({N_{T},Q})} = {\exp \left( {j\frac{2\pi}{N_{T}Q}{mn}} \right)}} \\{= {\exp \left( {j\frac{2\pi}{\frac{N_{T}}{2}\left( {2Q} \right)}{mn}} \right)}} \\{{= w_{m,n}^{({{N_{T}/2},{2Q}})}},}\end{matrix} & (13) \\{{n = 0},\ldots \mspace{14mu},{{QN}_{T} - 1},} & \;\end{matrix}$

while for the remaining antennas m=N_(T)/2+m′, m′=0, 1, . . . ,N_(T)/2−1,

$\begin{matrix}\begin{matrix}\begin{matrix}{w_{{{N_{T}/2} + m^{\prime}},n}^{({N_{T},Q})} = {\exp \left( {j\frac{2\pi}{N_{T}Q}\left( {{N_{T}/2} + m^{\prime}} \right)n} \right)}} \\{= {{\exp \left( {j\frac{2\pi}{\frac{N_{T}}{2}\left( {2Q} \right)}m^{\prime}n} \right)}{\exp \left( {j\frac{\pi}{Q}n} \right)}}} \\{= {w_{m^{\prime},n}^{({{N_{T}/2},{2Q}})}{\exp \left( {j\frac{\pi}{Q}n} \right)}}} \\{{= {w_{m^{\prime},n}^{({{N_{T}/2},{2Q}})}\alpha}},}\end{matrix} \\{{n = 0},\ldots \mspace{14mu},{{QN}_{T} - 1.}}\end{matrix} & (14) \\{{Here},{\alpha \in {\left\{ {{{{\exp \left( {j\frac{\pi}{Q}n} \right)}\text{:}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},{{2Q} - 1}} \right\}.}}} & \;\end{matrix}$

Any N_(T) element DFT precoder can thus be written as

$\begin{matrix}\begin{matrix}{w_{n}^{({N_{T},Q})} = \begin{bmatrix}w_{0,n}^{({N_{T},Q})} & w_{1,n}^{({N_{T},Q})} & \ldots & w_{{N_{T} - 1},n}^{({N_{T},Q})} & {w_{0,n}^{({N_{T},Q})}\alpha} & {w_{1,n}^{({N_{T},Q})}\alpha} & \ldots & {w_{{N_{T} - 1},n}^{({N_{T},Q})}\alpha}\end{bmatrix}^{T}} \\{= \begin{bmatrix}w_{n}^{({{N_{T}/2},{2Q}})} \\{w_{n}^{({{N_{T}/2},{2Q}})}\alpha}\end{bmatrix}} \\{= {{\begin{bmatrix}w_{n}^{({{N_{T}/2},{2Q}})} & 0 \\0 & w_{n}^{({{N_{T}/2},{2Q}})}\end{bmatrix}\begin{bmatrix}1 \\\alpha\end{bmatrix}}.}}\end{matrix} & (15)\end{matrix}$

However, this falls under the factorized precoder structure if thetuning precoder codebook contains the precoder elements

$\begin{matrix}{\left\{ {{{\begin{bmatrix}1 \\{\exp \left( {j\frac{\pi}{Q}n} \right)}\end{bmatrix}\text{:}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},{{2Q} - 1}} \right\},} & (16)\end{matrix}$

and moreover suits the closely spaced cross-polarized array perfectlybecause size N_(T)/2 DFT precoders are now applied on each antenna groupULA and the tuning precoder provides 2Q different relative phase shiftsbetween the two orthogonal polarizations. It is also seen how theN_(T)/2 element w_(n) ^((N) ^(T) ^(/2,2Q)) precoders are reused forconstructing the N_(T) element precoder w_(n) ^((N) ^(T) ^(,Q)).

Thus, as an example, the codebook 26 at the transceiver 10 and at thetransceiver 12 may be represented as two codebooks 70 and 72, such asshown in FIG. 7. In particular, the codebook 70 contains codebookentries comprising a defined set of conversion precoders 74, while thecodebook 72 contains codebook entries comprising a defined set of tuningprecoders 76. Pairings of respective conversion and tuning precoders 74and 76 form corresponding overall precoders 28. That is, the codebook 26at the transceiver 10 may comprise a set of overall precoders 28, eachrepresenting the combination of a selected conversion precoder 74 and aselected tuning precoder 76—e.g., the matrix product of the selectedconversion and tuning precoders 74 and 76. As such, it will beunderstood that the codebook 26 may be structured as a table or otherdata structure having overall precoders 28 as its entries, or mayequivalently be structured or represented by the conversion and tuningprecoder codebooks 70 and 72, respectively containing defined sets ofconversion and tuning precoders 74 and 76.

In at least one embodiment, the codebook 70 includes a number of DFTbased precoders as the conversion precoders 74. These DFT basedprecoders have an oversampling factor 2Q, which are used together withtuning precoders 76 in the codebook 72, for building overall precoders28 as DFT based precoders W with an oversampling factor Q for an antennaarray with twice as many elements. With this arrangement, theoversampling factor (2Q) is twice as large as for the co-polarized N_(T)element ULA (Q), but those elements are not wasted because they help toincrease the spatial resolution of the grid of beams precoders evenfurther. This characteristic is particularly useful in MU-MIMOapplications where good performance relies on the ability to veryprecisely form beams towards the UE of interest and nulls towards theother co-scheduled UEs.

For example, take a special case of N_(T)=8 transmit antennas—i.e.,assume that the transceiver 10 of FIG. 1 includes eight antennas 14, foruse in precoded MIMO transmissions, and assume that Q=2 for the closelyspaced ULA. One sees that the precoder is built up as

$\begin{matrix}\begin{matrix}{w_{n}^{({8,2})} = \begin{bmatrix}w_{n}^{({{N_{T}/2},{2Q}})} \\{w_{n}^{({{N_{T}/2},{2Q}})}\alpha}\end{bmatrix}} \\{{= {\begin{bmatrix}w_{n}^{({4,4})} & 0 \\0 & w_{n}^{({4,4})}\end{bmatrix}\begin{bmatrix}1 \\{\exp \left( {j\frac{\pi}{2}n^{\prime}} \right)}\end{bmatrix}}},}\end{matrix} & (17) \\{{n = 0},\ldots \mspace{14mu},{{2N_{T}} - 1},{n^{\prime} = 0},1,2,3.} & \;\end{matrix}$

The codebook 72 for the tuning precoders 76 can then be chosen from therank 1, 2 Tx codebook in LTE and hence that codebook can be re-used. Thecodebook for the conversion precoders 74 contains elements constructedfrom four DFT based generator matrices as in Eq. (8). The codebooks 70and 72 can contain other elements in addition to the DFT based ones.Broadly, this principle of constructing N element DFT precoders out ofsmaller, N/2 element DFT precoders can thus be used in general to addefficient closely spaced ULA and cross-pole support to a codebook basedprecoding scheme. Advantageously, this particular precoder structure canbe used even if the antenna setups differ from what is being assumed inthis example.

Further, note that DFT-based precoders can be used for transmissionranks higher than one, as well. One way to do so is to pick the antennagroup precoders 74 (denoted here as {tilde over (W)}^((c))) as columnsubsets of DFT-based generator matrices, such as those shown in Eq. (8).

The tuning precoders 76 can be extended with additional columns as well,to match the desired value of the transmission rank. For transmissionrank 2, a tuning precoder 76 can be selected as

$\begin{matrix}{{w^{(t)} = \begin{bmatrix}1 & 1 \\\alpha & {- \alpha}\end{bmatrix}},{\alpha \in {\left\{ {{{{\exp \left( {j\frac{\pi}{Q}n} \right)}\text{:}\mspace{14mu} n} = 0},1,\ldots \mspace{14mu},{{2Q} - 1}} \right\}.}}} & (18)\end{matrix}$

It is sometimes beneficial to re-use existing codebooks in the design ofnew codebooks. However, one associated problem is that existingcodebooks may not contain all the needed DFT precoder vectors to provideat least Q=2 times oversampling of the grid of beams. Assume for examplethat one has an existing codebook for N_(T)/2 antennas with DFTprecoders providing Q=Q_(e) in oversampling factor and that the targetoversampling factor for the N_(T)/2 element antenna group ULA isQ=Q_(t). The spatial resolution of the existing codebook can then beimproved to the target oversampling factor in factorized precoder designas

$\begin{matrix}{\mspace{79mu} {{{w = {\begin{bmatrix}{\Lambda_{\overset{\sim}{q}}w_{n}^{({{N_{T}/2},Q_{e}})}} & 0 \\0 & {\Lambda_{\overset{\sim}{q}}w_{n}^{({{N_{T}/2},Q_{e}})}}\end{bmatrix}\begin{bmatrix}1 \\\alpha\end{bmatrix}}},\mspace{79mu} {n = 0},\ldots \mspace{14mu},{{Q_{e}N_{T}} - 1},\mspace{79mu} {\overset{\sim}{q} = 0},1,\ldots \mspace{14mu},{{Q_{t}/Q_{e}} - 1}}{\Lambda_{\overset{\sim}{q}} = {{{diag}\begin{pmatrix}{1,{\exp \left( {j\frac{2\pi}{\frac{N_{T}}{2}}\frac{\overset{\sim}{q}}{Q_{t}}1} \right)},} \\{{\exp \left( {j\frac{2\pi}{\frac{N_{T}}{2}}\frac{\overset{\sim}{q}}{Q_{t}}2} \right)},\ldots \mspace{14mu},{\exp \left( {j\frac{2\pi}{\frac{N_{T}}{2}}\frac{\overset{\sim}{q}}{Q_{t}}\left( {{N_{T}/2} - 1} \right)} \right)}}\end{pmatrix}}.}}}} & (19)\end{matrix}$

Here, the w_(n) ^((N) ^(T) ^(/2,Qe)) could be elements in the existingLTE 4 Tx House Holder codebook, which contains 8 DFT based precoders(using an oversampling factor of Q=2) for rank 1. When the transmissionrank is higher than one, the block diagonal structure can be maintainedand the structure thus generalizes to

$\begin{matrix}{{w = {\begin{bmatrix}{\Lambda_{\overset{\sim}{q}}{\overset{\sim}{w}}^{(c)}} & 0 \\0 & {\Lambda_{\overset{\sim}{q}}{\overset{\sim}{w}}^{(c)}}\end{bmatrix}w^{(t)}}},} & (20)\end{matrix}$

where the overall precoder W is now an N_(T)×r matrix, the conversionprecoder {tilde over (W)}^((c)) is a matrix with at least one columnequal to a DFT based precoder w_(n) ^((N) ^(T) ^(/2,Qe)), and the tuningprecoder W^((t)) has r columns.

To see that that the spatial resolution can be improved by multiplyingthe antenna group precoder with a diagonal matrix as described above,consider the alternative parameterization of DFT precoders in Eq. (7),

$\begin{matrix}{{w_{m,{{Q_{t}l} + q}}^{({N_{T},Q_{t}})} = {\exp \left( {j\frac{2\pi}{N_{T}}{m\left( {l + \frac{q}{Q_{t}}} \right)}} \right)}},{m = 0},\ldots \mspace{14mu},{N_{T} - 1},{l = 0},\ldots \mspace{14mu},{N_{T} - 1},{q = 0},\ldots \mspace{14mu},{Q_{t} - 1},} & (21)\end{matrix}$

and let

$\begin{matrix}{{q = {{\frac{Q_{t}}{Q_{e}}q^{\prime}} + \; \overset{\sim}{q}}},{q^{\prime} = 0},\ldots \mspace{14mu},{Q_{e} - 1},{\overset{\sim}{q} = 0},\ldots \mspace{14mu},{\frac{Q_{t}}{Q_{e}} - 1},} & (22)\end{matrix}$

to arrive at

$\begin{matrix}\begin{matrix}{w_{m,{{Q_{t}l} + {\frac{Q_{t}}{Q_{e}}q^{\prime}} + \; \overset{\sim}{q}}}^{({N_{T},Q_{t}})} = {\exp \left( {j\frac{2\pi}{N_{T}}{m\left( {l + {\frac{1}{Q_{t}}\left( {{\frac{Q_{t}}{Q_{e}}q^{\prime}} + \; \overset{\sim}{q}} \right)}} \right)}} \right)}} \\{= {{\exp \left( {j\frac{2\pi}{N_{T}}{m\left( {l + \frac{q^{\prime}}{Q_{e}}} \right)}} \right)}{\exp \left( {j\frac{2\pi}{N_{T}}m\frac{\overset{\sim}{q}}{Q_{t}}} \right)}}} \\{= {w_{m,{{Q_{e}l} + \; q^{\prime}}}^{({N_{T},Q_{e}})}{\exp \left( {j\frac{2\pi}{N_{T}}m\frac{\overset{\sim}{q}}{Q_{t}}} \right)}}}\end{matrix} & (23) \\{{{{for}\mspace{14mu} m} = 0},\ldots \mspace{14mu},{N_{T} - 1},{l = 0},\ldots \mspace{14mu},{N_{T} - 1},{q^{\prime} = 0},\ldots \mspace{14mu},{Q_{e} - 1},{\overset{\sim}{q} = 0},\ldots \mspace{14mu},{\frac{Q_{t}}{Q_{e}} - 1.}} & \;\end{matrix}$

The above formulations demonstrate an advantageous aspect of theteachings presented herein. Namely, a codebook containing DFT precoderswith oversampling factor Q_(e) can be used for creating a higherresolution DFT codebook by multiplying the m:th antenna element with exp

$\left( {j\frac{2\pi}{N_{T}}m\frac{\overset{\sim}{q}}{Q_{t}}} \right)$

and hence proving that the diagonal transformation given byΛ_({tilde over (q)}) indeed works as intended. It is also conceivablethat such a structure where the antenna group precoder is multipliedwith a diagonal matrix in general (i.e., even when the codebooks are notusing DFT based vectors) can improve the performance.

As for the desirable properties of full PA utilization and rank nestedproperty, a first step in designing efficient factorized precodercodebooks while achieving full PA utilization and fulfilling the ranknested property is to make the conversion precoders 74 block diagonal asin Eq. (4). In a particular case, the number of columns k of aconversion precoder 74 is made equal to 2┌r/2┐, where ┌·┐ denotes theceil function. This structure is achieved by adding two new columnscontributing equally much to each polarization for every other rank. Inother words, such a conversion precoder 74 can be written in the form

$\begin{matrix}\begin{matrix}{w^{(c)} = \begin{bmatrix}{\overset{\sim}{w}}^{(c)} & 0 \\0 & {\overset{\sim}{w}}^{(c)}\end{bmatrix}} \\{{= \begin{bmatrix}{\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)} & 0 & 0 & \ldots & 0 \\0 & 0 & \ldots & 0 & {\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)}\end{bmatrix}},}\end{matrix} & (24)\end{matrix}$

where {tilde over (w)}_(l) ^((c)) is an N_(T)/2×1 vector.

Extending the conversion dimension in this manner helps in keeping thenumber of dimensions small and in addition serves to make sure that bothpolarizations are excited equally much. It is beneficial if theconversion precoder 74, {tilde over (W)}^((c)), is also made to obey ageneralized rank nested property in that there is freedom to choose{tilde over (W)}^((c)) with L columns as an arbitrary column subset ofeach possible {tilde over (W)}^((c)) with L+1 columns. An alternative isto have the possibility to signal the column ordering used in {tildeover (W)}^((c)). Flexibility in the choice of columns for {tilde over(W)}^((c)) for the different ranks is beneficial so as to still be ableto transmit into the strongest subspace of the channel even when rankoverride using a column subset is performed.

To ensure full PA utilization at the transceiver 10 in one or moreembodiments, one or more subsets 88 of the tuning precoders 76 areconstructed as follows: (a) the conversion vector {tilde over (w)}_(n)^((c)) is made constant modulus; and (b) a column in the tuning precoder76 has exactly two non-zero elements with constant modulus. If the m:thelement is non-zero, so is element m+┌r/2┐. Hence for rank r=4, thecolumns in an example tuning precoder 76 are of the following form

$\begin{matrix}{\begin{bmatrix}x \\0 \\x \\0\end{bmatrix},\begin{bmatrix}0 \\x \\0 \\x\end{bmatrix},} & (25)\end{matrix}$

where x denotes an arbitrary non-zero value which is not necessarily thesame from one x to another. Because there are two non-zero elements in acolumn, two orthogonal columns with the same positions of the non-zeroelements can be added before columns with other non-zero positions areconsidered. Such pair-wise orthogonal columns with constant modulusproperty can be parameterized as

$\begin{matrix}{\begin{bmatrix}1 \\0 \\^{j\; \varphi} \\0\end{bmatrix},{\begin{bmatrix}1 \\0 \\{- ^{j\; \varphi}} \\0\end{bmatrix}.}} & (26)\end{matrix}$

Rank nested property for the overall precoder is upheld when increasingthe rank by one by ensuring that columns for previous ranks excite thesame columns of the conversion precoder also for the higher rank.Combining this with Eq. (25) and the mentioned pair-wise orthogonalproperty of the columns leads to a double block diagonal structure ofthe tuning precoder 76 taking the form

$\begin{matrix}{W = {\begin{bmatrix}{\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)} & 0 & 0 & \ldots & 0 \\0 & 0 & \ldots & 0 & {\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)}\end{bmatrix}{\quad{\begin{bmatrix}x & x & 0 & 0 & \ldots \\0 & 0 & x & x & \; \\\vdots & \; & \; & \; & \ddots \\x & x & 0 & 0 & \ldots \\0 & 0 & x & x & \; \\\vdots & \; & \; & \; & \ddots\end{bmatrix}.}}}} & (27)\end{matrix}$

Using the pair-wise orthogonality property in Eq. (26), and representingthe precoder structure W as W^((c))W^((t)), the precoder structure canbe further specialized into

$\begin{matrix}{W = {\begin{bmatrix}{\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)} & 0 & 0 & \ldots & 0 \\0 & 0 & \ldots & 0 & {\overset{\sim}{w}}_{1}^{(c)} & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)}\end{bmatrix}{\quad{\begin{bmatrix}1 & 1 & 0 & 0 & \ldots \\0 & 0 & 1 & 1 & \; \\\vdots & \; & \; & \; & \ddots \\^{j\; \varphi_{1}} & {- ^{j\; \varphi_{1}}} & 0 & 0 & \ldots \\0 & 0 & ^{j\; \varphi_{2}} & {- ^{j\; \varphi_{2}}} & \; \\\vdots & \; & \; & \; & \ddots\end{bmatrix}.}}}} & (28)\end{matrix}$

Note that the double block diagonal structure for the tuning precoder 76can be described in different ways depending on the ordering of thecolumns in the conversion precoder 76. It is possible to equivalentlymake the tuning precoder 76 block-diagonal by writing

$\begin{matrix}{W = {\begin{bmatrix}{\overset{\sim}{w}}_{1}^{(c)} & 0 & {\overset{\sim}{w}}_{2}^{(c)} & 0 & \ldots & \ldots & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)} & 0 \\0 & {\overset{\sim}{w}}_{1}^{(c)} & 0 & {\overset{\sim}{w}}_{2}^{(c)} & \ldots & \ldots & 0 & {\overset{\sim}{w}}_{\lceil{r/2}\rceil}^{(c)}\end{bmatrix}{\quad{\begin{bmatrix}x & x & 0 & 0 & \ldots & \; & 0 & 0 \\x & x & 0 & 0 & \; & \; & \; & \vdots \\0 & 0 & x & x & \ddots & \; & \; & \; \\\vdots & \; & x & x & \; & \; & \; & \; \\\; & \; & 0 & 0 & \ddots & \; & \; & \; \\\; & \; & \vdots & \; & \; & \; & 0 & 0 \\\; & \; & \; & \; & \ddots & \; & x & x \\0 & 0 & 0 & 0 & \ldots & 0 & x & x\end{bmatrix}.}}}} & (29)\end{matrix}$

Re-orderings similar to these do not affect the overall precoder W andare thus to be considered equivalents falling with the meaning of theterms “block diagonal conversion precoder” and “double block diagonaltuning precoder,” as used herein. It is also interesting to note that ifthe requirements on the orthogonality constraint and full PA utilizationare relaxed, the design for rank nested property can be summarized withthe following structure for the tuning precoders 76

$\begin{matrix}{\begin{bmatrix}x & x & x & x & x & x & \; \\0 & 0 & x & x & x & x & \; \\\vdots & \; & \; & \; & x & x & \ddots \\x & x & x & x & x & x & \; \\0 & 0 & x & x & x & x & \; \\\vdots & \; & \; & \; & x & x & \ddots\end{bmatrix}.} & (30)\end{matrix}$

Finally, it is worth mentioning that rank nested property can be usefulwhen applied separately to the conversion precoders 74 and the tuningprecoders 76. Even applying it only to the tuning precoders 76 can helpsave computational complexity, because precoder calculations acrossranks can be re-used as long as the conversion precoder 76 remainsfixed.

As an illustrative example for eight transmit antennas 14 at thetransceiver 10, assume that Rank r=1

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & \; \\\; & w_{1}^{(1)}\end{bmatrix}\begin{bmatrix}1 \\^{j\; \phi_{k}}\end{bmatrix}}} & (31)\end{matrix}$

Rank r=2

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & \; \\\; & w_{1}^{(1)}\end{bmatrix}\begin{bmatrix}1 & 1 \\^{j\; \phi_{k}} & {- ^{j\; \phi_{k}}}\end{bmatrix}}} & (32)\end{matrix}$

Rank r=3

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & \; & \; \\\; & \; & w_{1}^{(1)} & w_{2}^{(1)}\end{bmatrix}\begin{bmatrix}1 & 1 & 0 \\0 & 0 & 1 \\^{j\; \phi_{k}} & ^{j\; \phi_{k}} & 0 \\0 & 0 & ^{j\; \phi_{l}}\end{bmatrix}}} & (33)\end{matrix}$

Rank r=4

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & \; & \; \\\; & \; & w_{1}^{(1)} & w_{2}^{(1)}\end{bmatrix}\begin{bmatrix}1 & 1 & 0 & 0 \\0 & 0 & 1 & 1 \\^{j\; \phi_{k}} & {- ^{j\; \phi_{k}}} & 0 & 0 \\0 & 0 & ^{j\; \phi_{l}} & {- ^{j\; \phi_{l}}}\end{bmatrix}}} & (34)\end{matrix}$

Rank r=5

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & \; & \; & \; \\\; & \; & \; & w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)}\end{bmatrix}{\quad\begin{bmatrix}1 & 1 & 0 & 0 & 0 \\0 & 0 & 1 & 1 & 0 \\0 & 0 & 0 & 0 & 1 \\^{j\; \phi_{k}} & {- ^{j\; \phi_{k}}} & 0 & 0 & 0 \\0 & 0 & ^{j\; \phi_{l}} & {- ^{j\; \phi_{l}}} & 0 \\0 & 0 & 0 & 0 & ^{j\; \phi_{m}}\end{bmatrix}}}} & (35)\end{matrix}$

Rank r=6

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & \; & \; & \; \\\; & \; & \; & w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)}\end{bmatrix}{\quad\begin{bmatrix}1 & 1 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 1 & 0 & 0 \\0 & 0 & 0 & 0 & 1 & 1 \\^{j\; \phi_{k}} & {- ^{j\; \phi_{k}}} & 0 & 0 & 0 & 0 \\0 & 0 & ^{j\; \phi_{l}} & {- ^{j\; \phi_{l}}} & 0 & 0 \\0 & 0 & 0 & 0 & ^{j\; \phi_{m}} & {- ^{j\; \phi_{m}}}\end{bmatrix}}}} & (36)\end{matrix}$

Rank r=7

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & w_{4}^{(1)} & \; & \; & \; & \; \\\; & \; & \; & \; & w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & w_{4}^{(1)}\end{bmatrix}{\quad\begin{bmatrix}1 & 1 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 1 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 1 & 1 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 1 \\^{j\; \phi_{k}} & {- ^{j\; \phi_{k}}} & 0 & 0 & 0 & 0 & 0 \\0 & 0 & ^{j\; \phi_{l}} & {- ^{j\; \phi_{l}}} & 0 & 0 & 0 \\0 & 0 & 0 & 0 & ^{j\; \phi_{m}} & {- ^{j\; \phi_{m}}} & 0 \\0 & 0 & 0 & 0 & 0 & 0 & ^{j\; \phi_{n}}\end{bmatrix}}}} & (37)\end{matrix}$

Rank r=8

$\begin{matrix}{W = {\begin{bmatrix}w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & w_{4}^{(1)} & \; & \; & \; & \; \\\; & \; & \; & \; & w_{1}^{(1)} & w_{2}^{(1)} & w_{3}^{(1)} & w_{4}^{(1)}\end{bmatrix}{\quad\begin{bmatrix}1 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 1 & 1 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 1 & 1 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & 1 & 1 \\^{j\; \phi_{k}} & {- ^{j\; \phi_{k}}} & 0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & ^{j\; \phi_{l}} & {- ^{j\; \phi_{l}}} & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & ^{j\; \phi_{m}} & {- ^{j\; \phi_{m}}} & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0 & ^{j\; \phi_{n}} & {- ^{j\; \phi_{n}}}\end{bmatrix}}}} & (38)\end{matrix}$

The 4 Tx case follows in a similar manner.

Another aspect of the teachings herein relates to the use ofsub-sampling of precoder codebook(s) for reducing the payload needed forprecoder selection feedback reporting. To illustrate how sub-sampling ofprecoder codebooks can be performed for reducing the payload size of theChannel State Information (CSI) reporting, consider the factorizedprecoder design discussed repeatedly herein. Namely, consider the casewhere an overall precoder W is represented by two precoders, each beingone of two factors used to represent W in factorized form. This approachwas seen earlier, with the use of a conversion precoder 74 denoted asW^((c)), and a tuning precoder 76 denoted as W^((t)). Thus, one has

$\begin{matrix}{W = {\begin{bmatrix}{\overset{\sim}{W}}^{(c)} & 0 \\0 & {\overset{\sim}{W}}^{(c)}\end{bmatrix}{W^{(t)}.}}} & (39)\end{matrix}$

If the codebook for the antenna group precoder {tilde over (W)}^((c))contains a set of DFT based precoders, then these precoders can besub-sampled by lowering the oversampling factor. This example ofsub-sampling results in only being able to use every K:th beam in thegrid of beams. Sub-sampling of the codebook can also be performed byselecting the M precoders out of N precoders in the codebook, whichmaximizes the minimum distance between the selected precoders on theGrassmanian manifold. Distances here can be measured, for example, asChordal distance, projection two-norm distance, or Fubini-Studydistance.

Sub-sampling principles as described above can also be applied to thetuning precoders 76 or to any precoder design. The sub-sampled codebookscan then be used on lower payload capable channels, e.g., PUCCH in LTE,while the full codebooks are used on the more capable ones, e.g., PUSCHin LTE. This approach can be understood as using sub-sampling to provide“coarse” CSI reporting on the PUCCH, while providing richer,higher-resolution CSI reporting on the PUSCH. For example, the use ofcodebook subset restrictions allows an LTE eNodeB to configure a UE toonly use a subset of precoders in a codebook, for computing andreporting CSI feedback (including the precoder selection feedback 44).

Further, some of the useful constraints on precoder selection by a UE orother targeted transceiver, such as precoders satisfying full PAutilization and rank nested property, might be too restrictive when itcomes to MU-MIMO operation, for example. This is because MU-MIMOoperation requires more freedom in the precoder design for increasedspatial resolution and resulting improved UE separation. Thus, oneaspect of the teachings herein is to reduce the signaling overhead forsignaling precoder restrictions by pre-defining one or more precodersubsets, with one subset designated for MU-MIMO and the other subsetdesignated for SU-MIMO. Signaling those subsets requires much lessoverhead since the individual precoders in each subset no longer need tobe explicitly signaled. Indeed, a single-bit flag can be used toindicate which one of the two subsets is to be considered by the UE as“currently permitted.”

In more detail, one of the predefined subsets 50 of overall precoders 28within the codebook 26 can be all overall precoders 28 that satisfy thefull PA utilization and rank nested property criteria. This subset wouldspecifically target SU-MIMO operation. Another subset 50 in the codebook26 could correspond to some or all of the overall precoders 28 in thecodebook 26, including those that violate full PA utilization and ranknested property. This latter subset 50 could be used for MU-MIMO, bothSU-MIMO and MU-MIMO. Moreover, the precoder subsets can be specifiedacross all ranks, or alternatively, precoder subsets together with ranksubsets can be signaled, for greater control from the network side.

When the overall precoders 28 in the codebook 26 are represented asdefined sets 80 and 86 of conversion and tuning precoders 74 and 76,subsets and subset restrictions may be defined separately for theconversion precoders 74 and the tuning precoders 76. Or, subsetrestrictions may be applied just to the conversion precoders 74, or justto the tuning precoders 76. In particular, it can be useful to applycodebook subset restrictions only to the conversion precoders 74. Thisis so because the conversion precoders 74 are the ones primarilyaffected by the MIMO mode in use (SU-MIMO or MU-MIMO).

Of course, the teachings herein are not limited to the specific,foregoing examples and accompanying illustrations. For example,terminology from 3GPP LTE was used in this disclosure to provide arelevant and advantageous context for understanding operations at thetransceivers 10 and 12, which were identified in one or more embodimentsas being an LTE eNodeB and an LTE UE, respectively. However, the use ofparameterized codebooks 26 may be used in other wireless systems,including but not limited to WCDMA, WiMax, UMB and GSM.

Further, the transceiver 10 and the transceiver 12 are not necessarily abase station and an item of mobile equipment within a standard cellularnetwork, although the teachings herein have advantages in such acontext. Moreover, while the particular wireless network examples givenherein involve the “downlink” from an eNodeB or other network basestation, the teachings presented herein also have applicability to theuplink. More broadly, it will be understood that the teachings hereinare limited by the claims and their legal equivalents, rather than bythe illustrative examples given herein.

1. A method in a wireless communication transceiver of controllingprecoder selection feedback sent to another wireless communicationtransceiver that precodes transmissions to the transceiver, wherein saidprecoder selection feedback indicates precoder selections by saidtransceiver, said method comprising: receiving restriction signalingfrom the other transceiver that identifies one or more permitted subsetswithin a defined set of overall precoders, or, where the defined set ofoverall precoders is represented by defined sets of conversion precodersand tuning precoders, said restriction signaling identifies one or morepermitted subsets of precoders within the defined sets of saidconversion precoders and tuning precoders; and generating said precoderselection feedback for sending to the other transceiver based onrestricting precoder selections by said transceiver according to saidrestriction signaling; wherein respective combinations of conversion andtuning precoders correspond to respective ones of the overall precoders.2. The method of claim 1, wherein a conversion precoder codebookcontains said defined set of conversion precoders and a tuning precodercodebook contains said defined set of tuning precoders, and wherein saidrestriction signaling indicates at least one of: a permitted subset ofconversion precoders in said conversion precoder codebook; and apermitted subset of tuning precoders in said tuning precoder codebook.3. The method of claim 2, wherein said precoder selection feedbackcomprises indications of a selected conversion precoder as selected bysaid transceiver from said permitted subset of conversion precoders insaid conversion precoder codebook, and a selected tuning precoder asselected by said transceiver from said permitted subset of tuningprecoders in said tuning precoder codebook.
 4. The method of claim 2,wherein said precoder selection feedback comprises index valuesidentifying codebook entries within said conversion and tuning precodercodebooks, as selected by said transceiver.
 5. The method of claim 1,further comprising at least one of: two or more predefined subsets ofconversion precoders and two or more predefined subsets of tuningprecoders; and wherein said restriction signaling indicates at least oneof: the permitted subset of conversion precoders among said two or morepredefined subsets of conversion precoders, and the permitted subset oftuning precoders among said two or more predefined subsets of tuningprecoders.
 6. The method of claim 5, wherein at least one of thepredefined subsets contains only precoders such that correspondingoverall precoders fulfill a full PA utilization property, while at leastone other one of the predefined subsets contains precoders such thatcorresponding overall precoders do not fulfill the full PA utilizationproperty; wherein the rows of an overall precoder matrix obeying thefull PA utilization property all have the same I²-norm, where I²-norm ofa row x with elements x_(k) is defined as$\sqrt{\sum\limits_{k}{x_{k}}^{2}}$
 7. The method of claim 5, whereinone or more of the predefined subsets are associated with a firstoperating mode of said transceiver or said other transceiver, while oneor more other ones of the predefined subsets are associated with asecond operating mode of said transceiver or said other transceiver, andwherein said restriction signaling identifies the one or more permittedsubsets of precoders by indicating which operating mode applies.
 8. Themethod of claim 1, wherein said defined set of conversion precoders orsaid defined set of tuning precoders are divided into one or more firstsubsets of precoders associated with Single-UserMultiple-Input-Multiple-Output, SU-MIMO, operation of the othertransceiver and one or more second subsets of precoders associated withMultiple-User MIMO, MU-MIMO, operation of the other transceiver, andwherein said restriction signaling identifies the one or more permittedsubsets of precoders by indicating whether SU-MIMO or MU-MIMO operationapplies.
 9. The method of claim 1, wherein said defined set ofconversion precoders includes N_(T)Q different conversion precoders andsaid defined set of tuning precoders includes a number of correspondingtuning precoders, and wherein each said conversion precoder comprises ablock diagonal matrix in which each block comprises a DFT-based precoderthat defines N_(T)Q different DFT based beams for a subgroup in a groupof N_(T) transmit antenna ports at the other transceiver, where Q is aninteger value and where the N_(T)Q different conversion precoders,together with one or more of the tuning precoders, correspond to a setof N_(T)Q different overall precoders, each overall precoder thusrepresenting a size-N_(T) DFT-based beam over the group of N_(T)transmit antennas ports.
 10. The method of claim 1, wherein the othertransceiver comprises a base station in a wireless communication networkand said transceiver comprises a user equipment, and wherein the methodfurther comprises receiving said restriction signaling at least in partas Radio Resource Control (RRC) layer signaling.
 11. A wirelesscommunication transceiver configured to control precoder selectionfeedback sent to another wireless communication transceiver thatprecodes transmissions to the transceiver, wherein said precoderselection feedback indicates precoder selections by said transceiver,said transceiver comprising: a receiver configured to receiverestriction signaling from the other transceiver that identifies one ormore permitted subsets within a defined set of overall precoders, or,where the defined set of overall precoders is represented by definedsets of conversion precoders and tuning precodes, said restrictionsignaling identifies one or more permitted subsets of precoders withinthe defined sets of said conversion precoders and said tuning precoders;and a precoding feedback generator configured to generate said precoderselection feedback for sending to the other transceiver, based onrestricting precoder sections by said transceiver according to therestriction signaling; wherein respective combinations of conversion andtuning precoders correspond to respective ones of the overall precoders.12. The transceiver of claim 11, wherein said transceiver stores arepresentation of a conversion precoder codebook containing said definedset of conversion precoders and a representation of a tuning precodercodebook containing said defined set of tuning precoders, and whereinsaid restriction signaling indicates at least one of: a permitted subsetof conversion precoders in said conversion precoder codebook; and apermitted subset of tuning precoders in said tuning precoder codebook.13. The transceiver of claim 12, wherein said precoder selectionfeedback comprises indications of a selected conversion precoder asselected by said transceiver from said permitted subset of conversionprecoders in said conversion precoder codebook, and a selected tuningprecoder as selected by said transceiver from said permitted subset oftuning precoders in said tuning precoder codebook.
 14. The transceiverof claim 12, wherein said precoding feedback generator is configured togenerate the precoder selection feedback as index values identifyingcodebook entries within said conversion and tuning precoder codebooks,as selected by said transceiver.
 15. The transceiver of claim 11,further comprising at least one of: said transceiver storing two or morerepresentations of predefined subsets of conversion precoders and two ormore representations of predefined subsets of tuning precoders; andwherein said restriction signaling indicates at least one of: thepermitted subset of conversion precoders among said two or morepredefined subsets of conversion precoders, and the permitted subset oftuning precoders among said two or more predefined subsets of tuningprecoders.
 16. The transceiver of claim 15, wherein at least one of thepredefined subsets contains only precoders such that correspondingoverall precoders fulfill a full PA utilization property for the othertransceiver, while at least one other one of the predefined subsetscontains precoders such that corresponding overall precoders do notfulfill the full PA utilization property; wherein the rows of an overallprecoder matrix obeying the full PA utilization property all have thesame I²-norm, where I²-norm of a row x with elements x_(k) is defined as$\sqrt{\sum\limits_{k}{x_{k}}^{2}}.$
 17. The transceiver of claim 15,wherein one or more of said predefined subsets are associated with afirst operating mode of said transceiver or the other transceiver, whileone or more other ones of the predefined subsets are associated with asecond operating mode of said transceiver or the other transceiver, andwherein said restriction signaling identifies the one or more permittedsubsets of precoders by indicating which operating mode applies.
 18. Thetransceiver of claim 11, wherein said defined set of conversionprecoders or said defined set of tuning precoders are divided into oneor more first subsets of precoders associated with Single-UserMultiple-Input-Multiple-Output, SU-MIMO, operation of the othertransceiver and one or more second subsets of precoders associated withMultiple-User MIMO, MU-MIMO, operation of the other transceiver, andwherein said restriction signaling identifies the one or more permittedsubsets of precoders by indicating whether SU-MIMO or MU-MIMO operationapplies.
 19. The transceiver of claim 11, wherein said defined set ofconversion precoders includes N_(T)Q different conversion precoders andsaid defined set of tuning precoders includes a number of correspondingtuning precoders, and wherein each said conversion precoder comprises ablock diagonal matrix in which each block comprises a DFT-based precoderthat defines N_(T)Q different DFT based beams for a subgroup in a groupof N_(T) transmit antenna ports at the other transceiver, where Q is aninteger value and where the N_(T)Q different conversion precoders,together with one or more of the tuning precoders, correspond to a setof N_(T)Q different overall precoders, each overall precoder thusrepresenting a size-N_(T) DFT-based beam over the group of N_(T)transmit antennas ports.
 20. The transceiver of claim 11, wherein theother transceiver comprises a base station in a wireless communicationnetwork and said transceiver comprises a user equipment, and whereinsaid transceiver is configured to receive said restriction signaling atleast in part as Radio Resource Control (RRC) layer signaling.
 21. Atransceiver configured to precode transmissions to another transceiverbased at least in part on receiving precoder selection feedback from theother transceiver that indicates an overall precoder for considerationby said transceiver in precoding to the other transceiver, saidtransceiver including a receiver for receiving said precoder selectionfeedback from the other transceiver and comprising: a precodingcontroller configured to determine a restriction that limits precoderselection by the other transceiver to one or more permitted subsets ofoverall precoders within a defined set of overall precoders, or to oneor more permitted subsets of precoders within defined sets of conversionprecoders and tuning precoders, wherein each overall precoder isequivalently represented by a unique combination of one conversionprecoder and one tuning precoder within the defined sets, and togenerate restriction signaling for indicating said one or more permittedsubsets to the other transceiver; and a transmitter cooperativelyassociated with said precoding controller and configured fortransmitting said restriction signaling to the other transceiver;wherein respective combinations of conversion and tuning precoderscorrespond to respective ones of the overall precoders in the definedset of overall precoders.
 22. The transceiver of claim 21, wherein saidother transceiver stores a representation of a conversion precodercodebook containing said defined set of conversion precoders and arepresentation of a tuning precoder codebook containing said defined setof tuning precoders, and wherein the transceiver is further configuredto generate the restriction signaling to indicate at least one of: apermitted subset of conversion precoders in said conversion precodercodebook; and a permitted subset of tuning precoders in said tuningprecoder codebook.
 23. The transceiver of claim 21, wherein saidprecoder selection feedback comprises indications of a selectedconversion precoder as selected by said transceiver from said permittedsubset of conversion precoders in said conversion precoder codebook, anda selected tuning precoder as selected by said transceiver from saidpermitted subset of tuning precoders in said tuning precoder codebook,and wherein said transceiver is configured to determine the overallprecoder from the defined set of overall precoders corresponding to saidindications in said precoder selection feedback.
 24. The transceiver ofclaim 21, wherein the transceiver maintains a codebook representation ofsaid defined set of overall precoders and uses indications of selectedconversion and tuning precoders conveyed by said precoder selectionfeedback to identify the overall precoder selected by the othertransceiver.
 25. The transceiver of claim 21, wherein said restrictionsignaling comprises a mode indicator that identifies one of a first anda second transmission mode, and wherein said precoding controller setsthe mode indicator in dependence on whether the transceiver is operatingin the first or the second transmission mode, and wherein one or moresubsets among said defined sets of conversion precoders and tuningprecoders at the other transceiver are permitted for said firsttransmission mode, and one or more other ones of the subsets among saiddefined sets of conversion precoders and tuning precoders at said othertransceiver are permitted for said second transmission mode
 26. Thetransceiver of claim 21, wherein said transceiver comprises a basestation in a wireless communication network.
 27. A method in a wirelesscommunication transceiver that is configured to precode transmissions toanother transceiver based at least in part on receiving precoderselection feedback from said other transceiver that indicates an overallprecoder for consideration by said transceiver in precoding to the othertransceiver, said method comprising: determining a restriction thatlimits precoder selection by the other transceiver one or more permittedsubsets of overall precoders within a defined set of overall precodersor to one or more permitted subsets of precoders within defined sets ofconversion precoders and tuning precoders, wherein each overall precoderis equivalently represented by a unique combination of one conversionprecoder and one tuning precoder within the defined sets; generatingrestriction signaling for indicating the one or more permitted subsetsto said other transceiver; and sending the restriction signaling to theother transceiver, to restrict precoder selection by the othertransceiver to the one or more permitted subsets; wherein respectivecombinations of conversion and tuning precoders correspond to respectiveones of the overall precoders in the defined set of overall precoders.28. The method of claim 27, wherein said other transceiver stores arepresentation of a conversion precoder codebook containing said definedset of conversion precoders and a representation of a tuning precodercodebook containing said defined set of tuning precoders, and whereinthe method further comprises said transceiver generating the restrictionsignaling to indicate at least one of: a permitted subset of conversionprecoders in said conversion precoder codebook; and a permitted subsetof tuning precoders in said tuning precoder codebook.
 29. The method ofclaim 27, wherein said precoder selection feedback comprises indicationsof a selected conversion precoder as selected by said transceiver fromsaid permitted subset of conversion precoders in said conversionprecoder codebook, and a selected tuning precoder as selected by saidtransceiver from said permitted subset of tuning precoders in saidtuning precoder codebook, and said transceiver determining the overallprecoder from the defined set of overall precoders corresponding to saidindications in said precoder selection feedback.
 30. The method of claim27, wherein said transceiver maintains a codebook representation of thedefined set of overall precoders and said method further comprises usingindications of selected conversion and tuning precoders conveyed by saidprecoder selection feedback to identify the overall precoder selected bythe other transceiver.
 31. The method of claim 27, wherein saidrestriction signaling comprises a mode indicator that identifies one ofa first and a second transmission mode, and wherein said method furthercomprises setting the mode indicator in dependence on whether thetransceiver is operating in the first or the second transmission mode,and wherein one or more subsets among said defined sets of conversionprecoders and tuning precoders at the other transceiver are permittedfor said first transmission mode, and one or more other ones of thesubsets among said defined sets of conversion precoders and tuningprecoders at the other transceiver are permitted for said secondtransmission mode
 32. The method of claim 27, wherein said transceivercomprises a base station in a wireless communication network.